This invention relates to an apparatus and method for cutting continuous webs into sheets, and, more particularly, to an improved knife roller assembly for cutting single-or multiple layer paper webs, while reducing web misfeeding and improving the separation of waste material from the cut sheets.
Many methods have been devised to facilitate the cutting of continuous webs. Typically, a rotating knife roll assembly cooperates with a stationary or rotating anvil assembly to periodically cut a sheet of predetermined length from the continuous web. The length of the sheet is established by the web feed rate and the distance between successive knives carried by the knife roll.
For efficient operation, the web must be fed and cut and the cut sheets must be conveyed, as fast as possible. However, higher web feed rates contribute to a greater potential of jamming, especially when the cut sheets must be fed into further apparatus. In particular, after the newly-formed leading edge of the web is cut and the sheet is further transported to subsequent feeding or processing stations, misfeeding of the leading edge can occur. This misfeeding results because the unsupported, rapidly-moving leading edge of the continuous web is deflected by the standing ambient air it passes through. The leading edge also tends to adhere to the knife edge and/or follow the curvature of the knife roll or anvil roll. The results of this misfeeding are an undesirable fold along a portion of the web (or a "dog ear") after the web leaves the knife roll assembly, or simply a jammed web.
Given the curved knife roll and its nearly abutting relation to the anvil roll or surface, the leading edge of the web is difficult to support as it is conveyed from the cutting nip. Air jets or vacuum ducts have been located just downstream of the cutting nip to help guide a trailing portion of the cut sheet as it leaves the cutting nip. In these prior art devices, the air jets or vacuum ducts acted on the trailing edge of the cut sheet to direct the trailing edge away from the knife edge and thereby encourage a clean separation of the cut sheet from the knife edge. Air jets have also been used to lift the trailing edge of the cut sheet for tucking and overlapping operations. Many such devices do not address the problem of feeding the cut sheets into subsequent apparatus downstream of the cutting nip at high speeds.
Also, in many applications, it is necessary not only to simply cut the continuous web to produce individual sheets, but also to trim and remove a portion of the web as scrap or waste. For example, where a border or blank portion occurs between the printed images on the web as a result of the spacing between rotary printing plates, and particularly where stripping pins or the like have marred the blank portion, it is advantageous to trim and remove the excess border or blank portion and sever the individual sheets at a single processing station. This can be done by providing two parallel cutting knives which sever a trim or waste strip between the trailing edge of the newly-cut sheet and the leading edge of the web.
However, the waste material created in this process can be difficult to remove, especially at higher web feed rates and where multiple layer webs are processed. Waste material has been removed by the use of vacuum ducts associated with the knife roll or anvil roll, which draw the waste material into or against the knife roll or anvil roll for transport away from the cutting operation. The vacuum ducts may draw the waste strips into and through the cylinder. Unfortunately, it appears that these devices would clog by accumulating waste strips. Vacuum ducts in the knife roll or anvil roll may hold the waste strip against the knife roll or anvil roll in situ to take it away from the cutting nip between those two rolls. The flow through the ducts is then reversed to push the waste strip away from the roll at a position circumferentially removed from the cutting site. However, a system having a reversible suction capability through the same orifice, depending solely on angular displacement, is quite complicated. Further, the ordinary vacuum ducts of such systems still would be incapable of disposing of a multi-layered waste strip. These ducts are only capable of directing suction to the inside layer of the waste strip, which is directly against the suction duct and substantially seals the duct off. Layers of the waste strip behind the inside layer simply are not exposed to the suction, now sealed by the inside layer of the waste strip, and are not usually successfully carried away from the cutting operation. Those layers of the waste strip which are not controlled can become entangled with the equipment or web and rapidly litter the area around the cutting machine.
The prior art also has employed pins on the knife roll to impale the area to be trimmed as waste material, i.e., a 3/8 inch nominal trim width, and carry it circumferentially away from the cutting nip. Stripper bars and ejector cams are used to remove trim strips from these pins, but may be ineffective because the waste material accumulates on the pins or gets caught in the stripper bar and clogs the machine. Further, the pins must be designed to avoid interference with the rotary knife, and tend to become less effective.
Additionally, jamming is sometimes created by an incomplete cut, especially at the second or downstream knife of a knife roll having two parallel knives. Although the first or upstream knife is generally able to cut the web cleanly due to the tension of the web, the downstream knife does not have the mechanical advantage of the web tension. Equipment operators tend to put the second knife down harder to compensate. The effect is a prematurely blunt second knife.
In sum, much of the apparatus mentioned here is inappropriate for reliable high-speed operation because the unsupported leading edge of a sheet or web tends to stick to a knife or to be diverted by the ambient air and thus misfed, or because it does not reliably dispose of the waste material cut from the web.